Separation of 1,2,4-trimethylbenzene from 1,2,3-trimethylbenzene by azeotropic distillation

ABSTRACT

1,2,4-Trimethylbenzene is difficult to sepparate from 1,2,3-trimethylbenzene because of the proximity of their boiling points. They are readily separated by azeotropic distillation. Effective agents are 1-propanol, methyl formate and 1-nitropropane.

FIELD OF THE INVENTION

This invention relates to a method of separating 1,2,4-trimethylbenzenefrom 1,2,3-trimethylbenzene using certain organic liquids as the agentin azeotropic distillation.

DESCRIPTION OF PRIOR ART

Azeotropic distillation is the method of separating close boilingcompounds or azeotropes from each other by carrying out the distillationin a multiplate rectification column in the presence of an added liquid,said liquid forming an azeotrope with one or more of the compounds to beseparated. Its presence on each plate of the rectification column altersthe relative volatility in a direction to make the separation on eachplate greater and thus require either fewer plates to effect the sameseparation or make possible a greater degree of separation with the samenumber of plates. The azeotrope forming agent is introduced with thefeed to a continuous column. The azeotrope forming agent and the morevolatile component are taken off as overhead product and the lessvolatile component comes off as bottoms product. The usual methods ofseparating the azeotrope former from the more volatile component arecooling and phase separation or solvent extraction.

The usual method of evaluating the effectiveness of azeotropicdistillation agents is the change in relative volatility of thecompounds to be separated. Table 1 shows the degree of separation orpurity obtainable by theoretical plates at several relativevolatilities. Table 1 shows that a relative volatility of at least 1.2is required to get an effective separation by rectification.

                  TABLE 1                                                         ______________________________________                                        Effect of Relative Volatility on Theoretical Stage                            Requirments.                                                                  Separation Purity,                                                                       Relative Volatility                                                Both Products                                                                            1.02   1.1     1.2 1.3  1.4 1.5  2.0 3.0                           (Mole Fraction)                                                                          Theoretical Stages at Total Reflux                                 ______________________________________                                        0.999      697    144     75  52   40  33   19  12                            0.995      534    110     57  39   30  25   14  9                             0.990      463    95      49  34   28  22   12  7                             0.98       392    81      42  29   22  18   10  6                             0.95       296    61      31  21   16  14   8   4                             0.90       221    45      23  16   12  10   5   3                             ______________________________________                                    

There are a number of commercial processes which produce complexmixtures of aromatic hydrocarbons in the cumene boiling range. Two ofthe close boiling found there are 1,2,4-trimethylbenzene and1,2,3-trimethylbenzene which boil only seven degrees apart. A process toseparate these two would enhance their value as pure compounds. Therelative volatility between these two is only 1.3 which makes itdifficult to separate them by conventional rectification

Azeotropic distillation would be an attractive method of effecting theseparation of these two if agents can be found that (1) will create alarge apparent relative volatility among these two and (2) are easy torecover from the compound. Table 2 shows the relative volatilityrequired to obtain 99% purity. With an agent giving a relativevolatility of 1.6 only 27 actual plates are required.

                  TABLE 2                                                         ______________________________________                                        Theoretical and Actual Plates Required vs. Relative                           Volatility for Terpene Separation                                             Relative                                                                              Theoretical Plates Required                                                                    Actual Plates Required                               Volatility                                                                            At Total Reflux, 99% Purity                                                                    75% Efficiency                                       ______________________________________                                        1.4     28               38                                                   1.3     22               30                                                   1.6     20               27                                                   ______________________________________                                    

OBJECTIVE OF THE INVENTION

The object of this invention is to provide a process or method ofazeotropic distillation that will enhance the relative volatility of1,2,4-trimethylbenzene from 1,2,3-trimethylbenzene in their separationin a rectification column. It is a further object of this invention toidentify organic compounds which in addition to the above contraints,are stable, can be separated from the 1,2,4-trimethylbenzene and recyledto the column with little decomposition.

SUMMARY OF THE INVENTION

The objects of this invention are provided by a process for separating1,2,4-trimethylbenzene from 1,2,3-trimethylbenzene which entails the useof certain organic compounds as the agent in azeotropic distillation.

DETAILED DESCRIPTION OF THE INVENTION

I have discovered that certain organic compounds will greatly enhancethe relative volatility between 1,2,4-trimethylbenzene and1,2,3-trimethylbenzene and permit separation by rectification whenemployed as the agent in azeotropic distillation. Table 3 lists thecompounds that I have found to be effective in separating1,2,4-trimethylbenzene from 1,2,3-trimethylbenzene by azeotropicdistillation. They are ethanol, 1-propanol, 1-butanol,2-methyl-1-butanol, 2-methyl-1-propanol, propyl acetate,4-methyl-2-pentanone, 3-methyl-2-butanone, 2-ethoxyethanol,1-methoxy-2-propanol, propylene glycol, tripropylene glycol,di-tert-butyl dicarbonate, methyl formate, ethyl lactate, methylpivalate, n-butyl propionate, ethyl ether, 1,4-dioxane, propylene glycolpropyl ether, 1-methoxy-2-propanol, mesityl oxide, butyronitrile,nitromethane, 1-nitropropane, 2-methoxyethanol and acetic acid.

                  TABLE 3                                                         ______________________________________                                        Effective Azeotropic Distillation Agents For                                  Separating 1,2,4-Trimethylbenzene From                                        1,2,3-Trimethylbenzene                                                                            Relative                                                  Compounds           Volatility                                                ______________________________________                                        None                1.3                                                       Ethanol             1.4                                                       1-Propanol          1.6                                                       1-Butanol           1.5                                                       2-Methyl-1-butanol  1.4                                                       2-Methyl-1-propanol 1.5                                                       Propyl acetate      1.4                                                       4-Methyl-2-pentanone                                                                              1.45                                                      3-Methyl-2-butanone 1.4                                                       2-Ethoxyethanol     1.45                                                      Propylene glycol    1.45                                                      Tripropylene glycol 1.45                                                      Di-tert-Butyl dicarbonate                                                                         1.4                                                       Methyl formate      1.5                                                       Ethyl lactate       1.5                                                       Methyl pivalate     1.45                                                      n-Butyl propionate  1.5                                                       Ethyl ether         1.45                                                      1,4-Dioxane         1.4                                                       Propylene glycol propyl ether                                                                     1.4                                                       1-Methoxy-2-propanol                                                                              1.6                                                       Mesityl oxide       1.5                                                       Butyronitrile       1.5                                                       Nitromethane        1.4                                                       1-Nitropropane      1.5                                                       2-Methoxyethanol    1.5                                                       Acetic acid         1.45                                                      ______________________________________                                    

THE USEFULNESS OF THE INVENTION

The usefulness or utility of this invention can be demonstrated byreferring to the data presented in Tables 1, 2 and 3. All of thesuccessful agents show that 1,2,4-trimethylbenzene can be separated from1,2,3-trimethylbenzene by means of azeotropic distillation in arectification column and that the ease of separation as measured byrelative volatility is considerable.

WORKING EXAMPLE

1. Fifty grams of a 1,2,4-trimethylbenzene - 1,2,3-trimethylbenzenemixture and 50 grams of 1-propanol were charged to a vapor-liquidequilibrium still and refluxed for five hours. Analyses indicated avapor composition of 84.4% 1,2,4-trimethylbenzene, 15.6%1,2,3-trimethylbenzene; a liquid composition of 77.2%1,2,4-trimethylbenzene, 22.8% 1,2,3-trimethylbenzene. This is a relativevolatility of 1.6.

I claim:
 1. A method for recovering 1,2,4-trimethylbenzene from amixture consisting of 1,2,4-trimethylbenzene and 1,2,3-trimethylbenzenewhich comprises distilling said mixture consisting of1,2,4-trimethylbenzene and 1,2,3-trimethylbenzene in the presence of anazeotrope forming agent, recovering the 1,2,4-trimethylbenzene and theazeotrope forming agent as overhead product and obtaining the1,2,3-trimethylbenzene as bottoms product, wherein said azeotropeforming agent consists of one material selected from the groupconsisting of ethanol, 1-propanol, 1-butanol, 2-methyl-1-butanol,2-methyl-1-propanol, propyl acetate, 4-methyl-2-pentanone,3-methyl-2-butanone, 2-methoxyethanol, 2-ethoxyethanol,1-methoxy-2-propanol, propylene glycol, tripropylene glycol,di-tert.butyl dicarbonate, methyl formate, ethyl lactate, methylpivalate, n-butyl propionate, ethyl ether, 1,4-dioxane, propylene glycolpropyl ether, mesityl oxide, butyronitrile, nitromethane, 1-nitropropaneand acetic acid.